1. Field of the Invention
The present invention relates to fixing devices for use in copying machines, printers and facsimile apparatus, and more particularly to devices for fixing toner images to recording media such as paper and OHP sheets, utilizing induction heating.
2. Description of the Related Art
Conventionally, copying machines and the like have incorporated therein a device for fixing a toner image to a sheet bearing the image as transferred thereto. The fixing device comprises, for example, a heat roller for melting the toner on the sheet, and a pressure roller in pressing contact with the heat roller for nipping the sheet therebetween. The heat roller is in the form of a hollow cylinder and includes a heater held on the center axis of the roller inside thereof by holder means. The heater comprises, for example, a halogen lamp and is adapted to generate heat with a specified voltage applied thereto. The heater is positioned on the center axis of the heat roller and therefore radiates heat uniformly on the roller inner wall, giving the roller outer wall a temperature distribution which is uniform circumferentially of the roller. The outer wall of the heat roller is heated until the temperature thereof reaches a level (e.g., 150.degree. to 200.degree. C.) suited to fixing. In this state, the heat roller and the pressure roller in contact therewith rotate in directions opposite to each other to nip the toner-bearing sheet between the rollers. At the portion of contact between the heat roller and the pressure roller (hereinafter referred to as the "nip"), the toner on the sheet is melted by the heat of the heat roller and fixed to the sheet. With the rotation of the two rollers, the sheet is thereafter transported by a discharge roller and delivered onto a discharge tray.
The fixing device described requires a relatively long period of time for the heat roller to reach the temperature suited to fixing after the power supply for the copying machine has been turned on. This entails the problem that the user, unable to use the copying machine in the meantime, is obliged to wait for a long time.
Accordingly, a fixing device of the induction heating type has been proposed as a device solving the problem (Japanese Laid-Open Patent Application No. 58-178385). This device comprises an open magnetic path iron core provided with a coil concentrically therewith and disposed inside a heat roller of metal conductor. A high-frequency current is passed through the coil which is proximate to the inner surface of the heat roller to set up a high-frequency magnetic field, which induces an eddy current in the heat roller, causing the roller itself to generate Joule heat by virtue of the skin resistance of the roller per se. The device of the induction heating type heats up more rapidly than the device including a halogen lamp or like heater since the heat roller of metal conductor itself generates heat.
However, since the heat roller of the fixing device described produces heat uniformly over the surface, the heat generated by the portion other than the nip for fixing is not used for fixing but is wasted, while an increased supply of current to heat up the nip rapidly results in increased heat generation at the other portion alike, giving rise to another problem in that the internal temperature of the copying machine rises. The result is further in conflict with the recent trend toward savings in energy.
The nip and the neighboring portion thereof are heated to nearly the same temperature by the heat generated over the entire wall area of the heat roller, so that the following problem also arises when the sheet is released from the nip with the rotation of the heat roller.
In the vicinity of the nip, the heat of the heat roller permits the toner to remain melted without fixing to the sheet. The molten toner is therefore liable to adhere to the heat roller, from which the toner is transferred to the sheet again, hence the problem of so-called offset.
Further because the toner remains melted without fixing to the sheet owing to the heat of the heat roller in the vicinity of the nip, the adhesion of the molten toner makes it difficult for the sheet to separate from the portion of the heat roller other than the nip. This leads to the problem that the sheet will wind around the heat roller to cause a jam.